Coagulation is the process of clotting, and the phrase "time of coagulation" generally means the time required for a small amount of blood to coagulate. The time of coagulation indicates the propensity of blood to coagulate. It can be determined, and was for many years, by collecting blood in a small container and merely observing elapsed time from the moment the sample was obtained to the time it coagulated. Clearly, this method was not very precise. Coagulation time may also be determined by collecting blood in a small capillary tube, then breaking off short pieces of the tube until threads of fibrin appear between the broken ends. Precision may still be questionable, but coagulation time measured by the latter method is normally six to seventeen minutes.
More precise analyzer technologies that are currently available in hospitals for the assessing coagulation condition are typically found in the hospitals' central laboratories. These analyzers: (1) are usually complex and difficult to operate, requiring trained laboratory technicians; (2) are usually bulky in size and weight; and (3) have moving mechanical parts. Another disadvantage of many available instruments and methods of analysis of whole blood coagulation parameters is that they require separation of plasma from the test sample prior to analysis. As a result, currently available instruments and methods, particularly those used by hospitals in their central laboratories, are not well suited for point of care analysis of whole blood coagulation parameters. Point of care equipment should be (1) compact; (2) simple enough for operation by nurses or even lay persons; and (3) rugged enough to withstand the rigors of transport from bedside to bedside and use in emergency situations.
U.S. Pat. No. 5,580,744 (Zweig) and U.S. Pat. No. 5,418,141 (Zweig et al.) disclose a test article and method for determining coagulation capability in a blood sample. The test article is a porous membrane having a coagulation initiator and substrate impregnated therein. In use, blood is applied to one face of the membrane and plasma is absorbed into the interior of the membrane in the presence of the coagulation initiator and substrate. The membrane produces a detectable signal (i.e., a fluorescent signal) for reading by an automated detector or test system including a timer and means for calculating a coagulation value. Although the disclosed test article is said to be suitable for use in a home setting, as alluded to above, the Zweig test article requires that the plasma be separated from the sample.
U.S. Pat. No. 4,547,735 (Kiesewetter et al.) discloses another example of known apparatus and methods for measuring and testing blood. The disclosed instrument includes two electrodes with which the conductivity of a sample may be measured. The sample contacting faces of the electrodes are placed in vertically separated horizontal planes in an accurately fixed spacing so a blood column of a given size is formed from a sample. Current acts on the sample and the instrument measures change in impedance to determine the hematocrit value of blood. There is no suggestion that the instrument could be used to assess other characteristics or qualities of blood. The instrument is expressly designed to ignore or avoid the effect of sedimenting or aggregating.
U.S. Pat. No. 5,601,995 (Exner) discloses an apparatus and method for detecting coagulation. The method includes providing a porous sheet and applying a blood sample to the sheet so that the blood spreads through a part of the sheet. The spreading extent or spreading rate of the blood in the sheet is visualized, or measured by measuring electrical conductivity across the sheet, electrical potential across the sheet or an electrical resistance of the sheet. In the visual or optical embodiment, one measures the extent of area covered by the sample prior to coagulation, or the rate of growth of the area of the spread sample and tries to extrapolate the time of coagulation from those observations. In the conductivity embodiment, electrodes are provided on either side of the porous sheet. The conductivity or electrical impedance between the electrodes depends on the wetted area between them and, as a sample spreads through the porous sheet, impedance is reduced and conductivity increased, thereby indicating the extent of spread in the sheet. In theory, coagulation has occurred when the rate of change in conductivity/impedance approaches zero. Obviously, in the visual or optical embodiment, one problem is that mere visual observation may not be very accurate. In either embodiment, the porous sheet may be affected by ambient conditions and handling, particularly severe problems if point of care use is attempted.
U.S. Pat. No. 5,298,224 (Plum) and U.S. Pat. No. 5,167,145 (Butler et al.) disclose apparatus for determining blood coagulation time using optical means. The Plum patent discloses the use of light and light detectors to measure blood transillumination and to determine coagulation time. The Butler et al. patent discloses the use of an infrared source and a photo detector to measure electromagnetic transmission and to determine coagulation time.
While the preceding patents reveal advances in the art of evaluating coagulation parameters or characteristics of blood, none discloses or provides an optimally durable, simple, portable and reliable apparatus and method for point of care use. Accordingly, there exists a need for such an analyzer and a method of analysis for accurately and reliably measuring coagulation characteristics of whole blood.